Add stale-scrape health alerting (email + in-app banner)

Silent scraper breakage (200-with-empty-body) went unnoticed for 16 days
because nothing flagged it. Now, after each daily direct scrape, a health
check compares every enabled competitor's freshest captured rate against a
staleness threshold (default 5 days, config: scrape_health_stale_days).

State lives in a new direct_scrape_health table so we alert on transitions
only — one email when a competitor goes stale, one when it recovers, never a
daily repeat. Email uses the stack's shared SMTP integration via
central_settings (recipient: scrape_health_alert_email, else SMTP reply_to/
from) — no new mail secrets in this app. The current stale set also drives an
in-app warning banner on the Direct Rates page (GET /direct/health), with an
on-demand re-check endpoint (POST /direct/health/check).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/services/mailer.py

@@ -0,0 +1,52 @@
+"""
+Minimal SMTP sender for the rates app.
+
+Reuses the stack's shared SMTP integration (managed once in the Settings
+app, same as the tasks app's mailer) via central_settings — no per-app mail
+secrets. Fire-and-forget: a mail failure must never break the scheduler.
+"""
+import logging
+import os
+import smtplib
+from email.message import EmailMessage
+
+from services.central_settings import get_integration_sync
+
+logger = logging.getLogger(__name__)
+
+HOTEL_NAME = os.getenv("VITE_HOTEL_NAME", "Hotel")
+
+
+def send_mail(to: str, subject: str, body: str) -> bool:
+    """Send a plain-text email through the central SMTP integration.
+    Returns True on success. Never raises — logs and returns False instead."""
+    if not to:
+        return False
+    cfg = get_integration_sync("smtp")
+    if not cfg or not cfg.get("host"):
+        logger.warning("SMTP not configured in central settings — email skipped")
+        return False
+
+    try:
+        msg = EmailMessage()
+        msg["From"] = cfg.get("from") or f"{HOTEL_NAME} Rates <noreply@localhost>"
+        msg["To"] = to
+        msg["Subject"] = subject
+        if cfg.get("reply_to"):
+            msg["Reply-To"] = cfg["reply_to"]
+        msg.set_content(body)
+
+        port = int(cfg.get("port") or 587)
+        with smtplib.SMTP(cfg["host"], port, timeout=15) as s:
+            if port != 465:
+                try:
+                    s.starttls()
+                except smtplib.SMTPException:
+                    pass  # server without STARTTLS (e.g. local relay)
+            if cfg.get("user"):
+                s.login(cfg["user"], cfg.get("pass") or "")
+            s.send_message(msg)
+        return True
+    except Exception as e:
+        logger.error(f"Email send failed: {e}")
+        return False